This invention relates generally to retinal prosthetics and more particularly to a method and apparatus for enhancing retinal prosthetic performance.
This invention relates to directly modulating a beam of photons of sufficient energy onto retinal prosthetic implants of patients who have extreme vision impairment or blindness.
A healthy eye is has photosensitive retinal cells (e.g. rods and cones) which react to specific wavelengths of light to trigger nerve impulses. Complex interconnections among the retinal nerves assemble these impulses which are carried through the optic nerve to the visual centers of the brain, where they are interpreted. Certain forms of visual impairment are primarily attributable to a malfunction of the photosensitive retinal cells. In such cases, sight may be enhanced by a retinal prosthesis implanted in a patient""s eye. Michelson (U.S. Pat. No. 4,628,933) and Chow (U.S. Pat. Nos. 5,016,633; 5,397,350; 5,556,423) teach a retinal implant, or implants, of essentially photoreceptors facing out of the eye toward the pupil, each with an electrode which can stimulate a bipolar, or similar, cell with an electrical impulse. This bipolar cell is acted upon by the electrical stimulus, to send appropriate nerve impulses essentially through the optic nerve, to the brain.
This invention is postulated as a necessary complement to this type of prosthesis, because the photoreceptors do not appear to be sensitive enough to the ordinary levels of light entering the eye in that not enough current is produced to sufficiently stimulate the retinal cells. Consequently, a light amplifier, or xe2x80x9chelperxe2x80x9d device would be needed. That device is the invention herein described, which also includes special characteristic implants.
Furness, et al. teach a xe2x80x9cvirtual retinal displayxe2x80x9d, U.S. Pat. No. 5,659,327, where xe2x80x9cThe virtual retinal display . . . utilizes photon generation and manipulation to create a panoramic, high resolution, color virtual image that is projected directly onto the retina of the eye . . . there being no real or aerial image that is viewed via a mirror or optics.xe2x80x9d Richard, et al. teach, U.S. Pat. No. 5,369,415, xe2x80x9c. . . a direct retinal scan display including the steps of providing a directed beam of light, modulating the beam of light to impress video information onto the beam of light, deflecting the beam in two orthogonal directions, providing a planar imager including an input for receiving a beam of light into the eye of an operator which involves a redirection diffractive optical element for creating a virtual image from the beam of light on the retina of the eye, and directing the beam of light scanned in two orthogonal directions and modulated into the input of the planar imager and the output of the planar imager into the eye of an operator.xe2x80x9d
Sighted individuals can use these devices above for their intended uses. However, they appear unsuitable for use by blind individuals with implanted retinal prosthetics of the photoreceptor-electrode kind. It would seem that they do not provide enough light power. Moreover, light amplitude cannot be arbitrarily increased because according to Slinly and Wolbarscht, Safety with Lasers and Other Optical Sources, the retinal threshold damage is 0.4 Joules per square centimeter.
The present invention is directed to a method and apparatus for providing enhanced retinal prosthetic performance. More particularly, the invention is directed to a light amplifier and electrical circuitry for driving an implanted retinal prosthesis to maximize electrical stimulation of the retinal nerves or cells, while avoiding damage thereto. The invention is also directed to improved implanted retinal prostheses, which maximize the advantages of the light amplifier.
In accordance with one aspect of the invention, light reflected from a viewed image (i.e., input image) is passed through a light amplifier to produce an output image which is applied to the photoreceptor array of a retinal prosthesis. The gain (or xe2x80x9ctransfer functionxe2x80x9d) of the light amplifier enables the photoreceptor array to drive output electrodes for producing retinal nerve impulses of sufficient magnitude to enhance perceived sight.
In accordance with another aspect of the invention, the light amplifier preferably compresses the range of light intensity, e.g., logarithmically, to enable maximum light amplification without overdriving the prosthetic photoreceptors.
In accordance with another aspect of the invention, the electrical stimulation of the retinal nerves is preferably pulsed, i.e., periodically interrupted to avoid any damage attributable to peak magnitude electrical signals. Periodic interruption can be implemented mechanically by a shutter periodically interrupting the light incident on the photoreceptor array and/or electrically via an appropriate wave shaping circuit.
In accordance with another aspect of the invention, the implanted prosthetic""s electrodes generate a sequence of positive and negative pulses to avoid producing a net charge in the eye. Successive pulses are preferably spaced in time by an interval xcex94t.
Four preferred embodiments are described. In accordance with the first embodiment of the invention a single wavelength is relied upon to activate a combined photodetector-electronics-electrode implanted unit which then produces a negative pulse, followed by a time delay, followed by a positive pulse. A photoreceptor implanted in the eye acts to produce an electrical stimulation with an equal amount of positive and negative charge. A single light wavelength is received by the photoreceptor. That single wavelength contains extractable energy. It also contains information, which may be encoded by amplitude modulation, frequency modulation, phase shift methods or pulse width modulation, for example. The photoreceptor activates an electrode with associated electronics. The electronics produces a negative pulse followed by a time delay followed by a positive pulse. A net charge of zero is introduced into the eye by the electrode-originating electrical pulses. The preferred delay time is in the range 0.1 millisecond to 10 milliseconds, with the delay time of 2 milliseconds is most preferred. When the retinal cell is not being electrically stimulated, it returns to a rest and recovery state. It is then in a state, electrically, that it was in prior to stimulation by the first electrical stimulation.
In accordance with the second embodiment, a first wavelength is used to stimulate a first set of xe2x80x9celectronicxe2x80x9d photoreceptors. These photoreceptors are connected so that the stimulation of the attached, or associated, electrodes results in a negative pulse. This negative pulse provides retinal cell stimulation. Then the shutter cuts in and stops light transmission to the eye. The retinal cell is in a rest and recovery state so that it returns, electrically, to the state it was in prior to stimulation by the first particular wavelength of light. A second particular wavelength of light then stimulates a second set of photoreceptors which are sensitive to that wavelength of light; while the first set of photoreceptors are not affected. This second set of photoreceptors is connected so that the stimulation of the attached, or associated, electrodes results in a positive pulse. The net charge introduced into the retinal cells must balance. So the positive charge introduced by the positive pulse must equal the negative charge introduced by the negative pulse. Again, the shutter cuts in and stops light transmission. Again, the retinal cells rest and recover and the process repeats. An aspect of the second embodiment is using an electro-optic, electronic or mechanical shutter to provide a period of no electrical stimulation to the retinal cells targeted for electrical stimulation.
In accordance with a third embodiment, which is a cross, so to speak, between the first and second embodiments two different wavelengths and two different types of diodes, each responsive to a corresponding wavelength are used. In this embodiment, one wavelength is used to pump in a high constant level of light to supply power to the electronics component. The other wavelength is used to send in information via amplitude, frequency, phase, pulse-width modulation, or combinations thereof. The stimulation pulse from the electronics to the electrode to the retinal cell is generated in a fashion similar to the pulses generated in the first embodiment, with a single wavelength.
A fourth embodiment is that of the logarithmic light amplifier itself, without any special implantable photoreceptors. This last embodiment may require a low duty cycle when used with photoreceptors connected to a diode without any electronics. It may be able to rely sufficiently upon the intrinsic capacitance of an oxidizable electrode, which acquires capacitance with the buildup of an insulating oxidized layer toward the ionizable fluid present in the eye as vitreous fluid, or fluid directly associated with the eye.
An image receiver with a first converter for the image, converts the image into electrical signals. The signals are amplified, basically logarithmically, so as to provide brightness compression for the patient.
An aspect of the embodiments of the invention is that an amplified electrical signal is converted by a second converter into a photon-based display; the photons of this display enter an eye through the pupil of the eye. Moreover, while the embodiments of the logarithmic amplifier invention have sufficient output light power, advantageously, the output light level still remains at a safe level. This aspect of the invention corresponds to aspects of the action of the iris, as well as the biochemistry of retinal cells, in the human eye in making possible sight over many orders of magnitude of ambient brightness.
An aspect of the embodiments of the invention is incorporation of both optical and electronic magnification of the image, as for example, the incorporation of an optical zoom lens, as well as electronic magnification. Consequently, it is feasible to focus in on items of particular interest or necessity.
With proper adjustment, proper threshold amplitudes of apparent brightness would obtain, as well as comfortable maximum thresholds of apparent brightness. Therefore, to adjust for these, an adjustment aspect is incorporated in each embodiment, such that proper adjustment for the threshold amplitudes and maximum comfortable thresholds are made.
Another aspect of the invention, which may be incorporated in all embodiments, is oriented toward making color vision available, at least to a degree. To the extent that individual stimulation sites (e.g., retinal cells generally, bipolar cells specifically) give different color perceptions upon stimulation, the color of selected pixels of the viewed scene is correlated with a specific photoreceptor-electronics-electrode units located so as to electrically stimulate a specific type of bipolar cell to provide the perception of color vision.
In order to help implement both comfortable adjustment of threshold and maximum brightness, and color vision, the logarithmic light amplifier also incorporates within itself, a data processing unit which, semi-autonomously, cycles through the various photodetector-electrode and combinations thereof, interrogates the patient as to what the patient sees, the patient then supplies the answers, setting up proper apparent brightness, proper apparent color and proper perception. This setup mode is done by the use of a keyboard, display, and auxiliary processor, which are plugged into the data processing unit of the logarithmic light amplifier during the setup procedure.
A scanning laser feedback is provided in different embodiments of the invention to keep the scanner laser scanning the correct locations. An imaging of the reflected scanning laser reflected back from the retinal implant is used to provide real-time feedback information, utilizing a second imager viewing into the eye and a data processor unit tied into the scanning laser scan control unit.